1. Field of the Invention
The present invention relates to an optical data recording and reproducing apparatus (hereinafter referred to as an optical disc apparatus) which records optical data onto an optical recording medium, such as a magnetic optical disc, and reproduces the recorded data from the optical recording medium. More precisely, it relates to an improved carriage (optical head) which is moved in the tracking direction across data recording tracks, formed circumferentially on the optical disc thereof, to record and reproduce optical data.
2. Description of Related Art
In a known optical disc apparatus, such as a magnetic optical disc apparatus, an objective lens is supported by an objective lens holder which is movable in both the focusing direction and the tracking direction of the optical disc on a carriage. Namely, the movable objective lens holder, on which the objective lens is supported, is provided with focusing and tracking coils which, together with a magnetic circuit provided on the carriage constitute an electromagnetic drive mechanism. The carriage holds a mirror which reflects laser light emitted from a stationary optical system in the radial direction of the optical disc toward the objective lens. The focusing and tracking coils are supplied with a predetermined intensity and direction of electric current to precisely converge the laser light onto a predetermined recording track of the optical disc to thereby control the focusing and tracking operations.
In the known optical disc apparatus mentioned above, it is necessary to detect the position of the objective lens (lens holder), particularly in the radial direction (tracking direction) of the optical disc, to thereby determine a deviation from a correct position and reduce access time. To this end, a detector, which uses a photosensor, is incorporated in the optical disc apparatus.
The detector comprises, for example, a pair of reflectors (reflecting plates) provided on opposite sides of the lens holder in the tracking direction, and reflection type photosensors, opposed to the reflectors and secured to the carriage. Each photosensor has a pair of light emitting and receiving elements, so that the radial position of the objective lens can be detected in accordance with a difference in output between the two photosensors, depending on the displacement of the objective lens in the radial direction (tracking direction), to thereby control the position of the objective lens.
However, in the conventional optical disc apparatus, it is necessary to provide the photosensors in the vicinity of the objective lens. Accordingly, light reflected by the optical disc is made incident upon the photosensors and causes an error of measurement by the photosensors.
Furthermore, if the characteristics of the photosensors are identical, the photosensors must be positioned at equal distances from the reflectors of the lens holder. This however makes it difficult to independently adjust the position of the photosensor, however.
In addition to the foregoing, it is premised that the reflection type photosensors have an identical output characteristic, but, in practicality, it is very difficult to realize identical output characteristics in the two photosensors. If the output characteristics of the two photosensors are not identical to each other, there will be a measurement error in the detection of the position of the objective lens using the output difference of the two photosensors. Consequently, upon initial adjustment of the position of the photosensors, in accordance with the outputs thereof, the distances of the photosensors from the lens holder are not identical to each other. To compensate for this, it is necessary to independently adjust the distances of the photosensors from the reflectors provided on the lens holder. However, this is troublesome and complicated.